Such devices are primarily used in interventional or surgical procedures in which one or more 2D radioscopy images of the interesting area of the patient are recorded and displayed using x-ray fluoroscopy, for example in interventional radiological procedures, in interventional electrophysiological procedures or in procedures in interventional cardiology.
A frequent problem in these procedures is the spatial orientation of the doctor performing the procedure. Fluoroscopy images not only have the disadvantage of being limited to two-dimensional imaging, but—unless a contrast agent is used—only provide relatively little information as regards tissue structures. Soft tissue structures in particular cannot be mapped in fluoroscopy images, or only insufficiently. For this reason so-called image merger methods are already used, in which three-dimensional (3D) image data is combined with the fluoroscopy images and displayed together. The 3D images can for example be image data from computer tomography, magnetic resonance tomography or 3D rotation angiography, which were recorded from the area of interest of the examination object. These image merger methods are also known as 3D superimposition, 2D-3D superimposition, 2D-3D registration or 3D roadmapping. A prerequisite for joint or superimposed image display is however a correspondence in the perspective and scaling of the superimposed images.
Until now only really simple display methods are known. Thus in the “iPilot” product from Siemens, which is shown for example in the flyer “syngo iPilot—Effective guidance during interventional procedures”, 2005/11, Siemens Medical Solutions, Order No. A91AX-20004-11C-1-76, the display takes place for example in the following steps. A two-dimensional view is generated from the 3D data record by means of volume rendering and creates the impression of a 3D image, so-called perspective volume rendering being employed which uses precisely the geometry of the x-ray system as mapping geometry for recording the x-ray fluoroscopy image. The x-ray focus here corresponds to the eye and the detector of the image plane. Following this each pixel of the two-dimensional volume rendering display can be assigned a pixel of the fluoroscopy image. The two images are mixed to create a joint image display, i.e. it is possible to cross-fade between the two images.
In most angiography systems however the cone angle of the x-ray geometry is relatively small and generally lies only between 10 and 20°, so that the corresponding perspective volume rendering looks very similar to the volume renderings usually created in parallel geometry. This usual volume rendering does not represent the optimum display method for some applications. In particular for displaying hollow organs another display method has become common, known as virtual endoscopy, also called fly display. In virtual endoscopy the eye of the observer is directed to the inside of the object and a perspective geometry with a very large viewing angle of typically 90° to 180° is selected. This type of display produces a very natural effect primarily for electrophysiological procedures and is the preferred method, for example for observing the pulmonary vein orifices.
However, a problem arises if such virtual endoscopy views are to be merged with 2D radioscopy images, especially with fluoroscopy images. A simple superimposition as in the past is not possible, because the mapping geometry is different and hence there is no direct referencing between two points in the two images.